Support spawning vanilla gases from explosive projectile detonation

Add support for the postExplosionGasType XML tag in
CompProperties_ExplosiveCE, which allows our explosive projectiles to
spawn a built-in gas type on detonation. We seem to be already using it
in XML, the code just was not ready to use it.

Source/CombatExtended/CombatExtended/Comps/CompExplosiveCE.cs

@@ -53,7 +53,7 @@ public virtual void Explode(Thing instigator, Vector3 pos, Map map, float scaleF
                     Props.postExplosionSpawnThingDef,
                     Props.postExplosionSpawnChance,
                     Props.postExplosionSpawnThingCount,
-                    postExplosionGasType: null,
+                    Props.postExplosionGasType,
                     Props.applyDamageToExplosionCellsNeighbors,
                     Props.preExplosionSpawnThingDef,
                     Props.preExplosionSpawnChance,
@@ -67,4 +67,4 @@ public virtual void Explode(Thing instigator, Vector3 pos, Map map, float scaleF
             }
         }
     }
-}
+}

Source/CombatExtended/CombatExtended/Comps/CompProperties_ExplosiveCE.cs

@@ -34,6 +34,11 @@ public class CompProperties_ExplosiveCE : CompProperties
         public bool damageFalloff = true;
         public float chanceToStartFire;
 
+        /// <summary>
+        /// The type of built-in core game gas to spawn on detonation.
+        /// </summary>
+        public GasType? postExplosionGasType;
+
         public CompProperties_ExplosiveCE()
         {
             compClass = typeof(CompExplosiveCE);